The invention relates to flashing lamps and deals more particularly with a fluorescent lamp and associated electronic unit which causes the lamp to periodically flash. The flashing lamp may be used to illuminate a target which is being viewed by a video camera in a system where the camera and target are moving relative to one another, such as, for example, in a line follower system.
Previously, Xenon lamps have been used as a source of flashing light and within such lamps are gas and two spaced apart electrodes. To illuminate a Xenon lamp, a large voltage of the order of several kilovolts is applied between the electrodes and this voltage causes a current to arc across the gas between the electrodes, and this arc directly provides a source of light. Xenon lamps are available in spherical and linear shapes and possibly others. In a typical spherical lamp, the electrodes are spaced apart by less than an inch. Consequently, the light produced by an arc from such a spherical Xenon lamp is highly concentrated and will likely cause uneven illumination of a target. This problem is serious in a typical line follower system in which an optical scanning device, light source, and associated mechanical parts are located very close to one another and to a source of graphic material because such a close arrangement leads to a significant amount of reflections and shadows. Also, a light source which provides symmetrical illumination is especially important when the target is three-dimensional because such targets are inherently prone to shadowing. It is possible to use a defocusing lens in front of such a Xenon lamp to disperse the light more uniformly over the target area. However, for certain video applications the illumination field still contains illumination variations.
Linear or "pencil" lamps of the Xenon variety are available with electrodes spaced apart by three-four inches, and by virtue of the increased spacing and resultant arc length, such lamps provide a more dispersed source of light and a more uniform illumination of a large target than possible with the spherical variety described above.
To improve the uniformity of illumination it is helpful if the source of light is symmetrical relative to the target area and a viewing axis or line of sight of an optical scanning device, and with a single, spherically shaped lamp of any variety it is difficult to provide such symmetry because to do so may require that the scanning device be located in front of the lamp and this arrangement would block much of the light to a target area near the front of the optical scanning device.
To provide a more symmetrical source of illumination than that provided by a single, spherically shaped Xenon lamp, a plurality of such Xenon lamps can be utilized, evenly spaced around a viewing axis of an optical scanning device, the more lamps the more uniform the resultant illumination. However, the additional lamps increase the cost of the light source, requirements for power, and production of heat.
As an alternate source of relatively uniform illumination, two linear Xenon lamps, such as described above, may be arranged parallel to one another with an optical device situated such that its viewing axis passes between the pencil lamps.
In a system in which a target is being viewed by a video camera and the target and video camera are moving relative to one another, a stroboscopic light source may be used to "freeze" an image of the target so that the camera can generate a series of acceptably clear pictures. To periodically flash one or more Xenon lamps of the spherical or linear variety in such a stroboscopic manner, a short burst of electrical energy may be applied to the lamp periodically as, for example, by periodically applying a pulse of voltage between the associated electrodes. An ionization electrode used in conjunction with a linear Xenon lamp may be located along the shell of the lamp near the gap between the other two electrodes within the lamp and electrically shorted to one electrode within the lamp, and such an additional electrode used in conjunction with a spherical Xenon lamp may be located within the lamp in the gap between the other two associated electrodes. When an excitation voltage is applied between the two electrodes, whin either variety of Xenon lamp, and to the extra electrode, the extra electrode aides in ionizing the gas within the lamp.
Shorter flashes of light, less than five hundred microseconds each, may preferred in the video "frame-grabbing" or stroboscopic application described above.
Flourescent bulbs on the other hand are typically used as a more continuous source of light such as for room lighting or desk lighting purposes. A flourescent lamp ordinarily comprises a glass tube, two filaments serving as two cathodes or a cathode and an anode, one filament at each end of the tube, a coating of powdered phosphor on the inside of the tube, and argon gas and a small amount of mercury within the tube. To illuminate the lamp, an operator usually closes a switch which causes the cathode to be heated by an electrical current to provide a source of free electrons. Then, by means of a starter coil and choke, a large voltage is applied between the anode and the cathode which causes the argon gas to ionize and soon afterwards causes the mercury to conduct between the two filaments. As the mercury conducts, it radiates ultraviolet light which is absorbed by the phosphor on the inner surface of the tube and in response, the phosphor re-radiates light in the visible spectral range to provide illumination.
Using an ordinary fluorescent lamp excited in the manner above, it is abnormal to switch the lamp on and off very frequently because the relatively high voltage and current needed to begin the conduction and illumination process has an adverse impact on a coating on the filament. See Science and Invention Encyclopedia, H.S. Stuttman Co., Inc. Publisher, New York, Vol. 6, Page 768. Also, such an excitation system even if switched on and off automatically cannot cause a fluorescent lamp to flash for very short durations such as required in some video frame-grabbing or other stroboscopic applications.
Accordingly a general aim of the invention is to provide a substantially uniform and periodic source of flashing light for an optical scanning device.
Another aim of the invention is to utilize a fluorescent lamp as a source of flashing light.
A more specific aim of the invention is to provide a substantially uniform light source which is periodically flashed for a duration of less than one millisecond and preferably shorter.
Still another aim of the invention is to provide a flashing lamp unit which provides the illumination uniformity and short flash duration described above yet has acceptable cost, heat dissipation, and ultraviolet light radiation characteristics.
Yet another aim of the invention is to provide a flashing lamp unit comprising a fluorescent lamp which is housed in such a way that it easily can be replaced after it burns out.